Vehicles are powered by internal combustion engines which operate on gasoline mixed with air in a carburetor. Vacuum from the engine draws gasoline through a venturi structure for atomization. The gasoline and air mixture is then drawn into the engine for ignition.
Demands for fuel efficiency and anticipated scarcity of liquid fuels derived from petroleum has led to a search for other fuels which can be used instead of gasoline. Some potential alternatives include gaseous, volatile fuels such as propane, butane, methane, compressed or liquified natural gas.
The use of vehicles operating on alternate fuels, natural gas or LP gas as the primary fuel, with gasoline being the alternate fuel, should be limited to operations where the equipment is lightly loaded such as taxicabs, delivery vans, pickups, etc. The inherent loss of volumetric efficiency of the engine, due to the vapor density difference between the gaseous fuels and gasoline as well as the heat input required in the intake manifold for gasifying the gasoline, reduces the performance reserve at the maximum engine demand.
Many simple, small gasoline carburetors can be altered to receive gaseous fuels without upsetting the gasoline operation. More complex systems such as two-barrel and four-barrel carburetors, usually require the use of adapters. The use of solenoids on both the gasoline line and the gaseous fuel line with a selector switch in the driver compartment provides a simple convenient means for the operator to change from one fuel to the other.
To allow the operation of the alternate fuel system (gasoline) with a minimum adverse effect on the system, the adapters require large air capacities. The venturi signals from such adapters are quite low and require either a very sensitive fuel device for starting and idling or complex support systems to provide these functions.